Quantitative knowledge of the structure of the cell has great importance for biomedical application. However, conventional light microscopy does not provide sufficient quantitative information on light propagation within sub-cellular organelles of a biological cell. For example, cancer is typically diagnosed on the basis of morphological changes, particularly alterations in nuclear structure, in the cell. Nuclear architectural alterations are examined by bright-field microscopy of cells stained with reagents such as Papanicolaou, Diff-Quik, Hematoxylin and Eosin stains; the observable changes in malignant cells include enlarged nuclear size, irregularity of nuclear shape and more prominent nucleoli. However, subtle changes in the nuclear architecture may not be easily detectable with conventional microscopy, especially in early stages of cancer development, which can delay definitive diagnosis until the malignant features of nuclear architectures become significant. While nanoscale changes in nuclear architecture can be assessed by electron microscopy, this approach is expensive, labor intensive, and requires specialized tissue or cell handling that can damage the sample and make it impractical for routine clinical use. Similarly, although optical phase microscopy uses the ultra-sensitivity of light interference effect to detect sub-cellular changes in architecture, quantitative phase imaging microscopy suffers from speckle noise that has significantly hampered its clinical utility. Thus, clinically practical techniques are needed to improve early detection of cancer, provide insight into the process of malignant transformation, and inform development of new diagnostic tools and therapeutic agents.
Phase contrast microscopy and differential interference contrast (DIC) microscopy are capable of detecting subtle subcellular structural alterations. They have been widely used to visualize transparent cells in biological research, in which a minute alteration in the phase or optical path length of internal cell structure, even just a few protein or DNA molecules, can be detected through the intensity differences in the image. Despite their ability to visualize transparent cells, the lack of quantitative phase information has become a limiting factor in many biological applications. Due to the significant technical advancement, quantitative phase microscopy has recently emerged as a superior phase microscopy technique, as it provides quantitative phase measurement of a biological cell with ultrasensitivity in detecting subtle dynamic changes in the subcellular structure. Despite these significant advances, its utility in clinical diagnosis of cancer is still limited, largely due to the speckle noise, special requirement on sample preparations, and the lack of known diagnostic parameters for cancer.